MIPS: cmpxchg: Drop __xchg_u{32,64} functions

MIPS: cmpxchg: Drop __xchg_u{32,64} functions

The __xchg_u32() & __xchg_u64() functions now add very little value.
This patch therefore removes them, by:

  - Moving memory barriers out of them & into xchg(), which also removes
    the duplication & readies us to support xchg_relaxed() if we wish to.

  - Calling __xchg_asm() directly from __xchg().

  - Performing the check for CONFIG_64BIT being enabled in the size=8
    case of __xchg().

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16352/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>